A magnetic sensor using a Hall element is widely used not only as a proximity sensor, a linear position sensor, a rotation angle sensor, or the like, each of which is a sensor for detecting positional information of a magnet, but also as a current sensor for measuring, in a non-contact manner, the amount of current flowing in a current conductor by detecting a magnetic field induced by the current flowing in the current conductor.
Particularly, it is required for the current sensor used to detect an inverter current of a motor, to highly precisely detect the inverter current that is switched at a high-speed frequency, for the purpose of improving the efficiency of motor control.
Since these types of Hall elements have a magnetoelectric conversion function to generate a Hall electromotive force signal depending on an intensity of an input magnetic field, the Hall element is widely used as a magnetic sensor. However, an offset voltage (unbalanced voltage) exists in the Hall element. The offset voltage is a non-zero finite voltage which is output even in a state where the magnetic field is zero, i.e., in a state of no magnetic field.
Therefore, in regard to the magnetic sensor using a Hall element, in order to cancel the offset voltage of the Hall element, there is a driving method for the Hall element, which is generally known as the Spinningcurrent method or the Connection commutation method. This method includes periodically switching between the positions of a terminal pair for flowing a driving current to the Hall element and the positions of a terminal pair for detecting a Hall electromotive force signal according to a clock called as a chopper clock (see NPL1 and PTL2, for example).
The Spinningcurrent method for the purpose of cancelling the offset voltage is configurable by use of a switch circuit in a CMOS semiconductor circuit, and therefore, a Hall electromotive force detecting circuit to obtain a highly-accurate magnetic sensor generally includes the switch circuit to implement the Spinningcurrent method.
Furthermore, in a case where the magnetic sensor utilizing the Hall element for measuring the current of the inverter is used, a wide-band property regarding a signal band, a high-speed response property regarding a signal processing delay time, a low-noise property regarding a signal quality, and the like are required for the magnetic sensor. Therefore, as a circuit system for processing the Hall electromotive force signal generated in the Hall element in these cases, a continuous-time signal processing circuit which processes signal in the continuous time is more advantageous than a discrete-time signal processing circuit which time-discretizes (sampling). In the continuous-time signal processing circuit, an aliasing phenomenon of the noise due to the time-discretization (sampling) is caused. Thus, the continuous-time signal processing circuit is especially suitable for use in an environment with much high frequency noise due to switching of the inverter.
The Spinningcurrent method of the Hall element will be described below with reference to FIGS. 1A and 1B.
FIGS. 1A and 1B are views illustrating the Hall electromotive force detection when the direction of the driving current biasing the Hall element is switched between 0 degree and 90 degrees every time the phase of the chopper clock is switched between two values, i.e., φ1 and φ2. The Hall element in the drawings is modeled as a four-terminal element including four resistors (R1, R2, R3, R4), and is driven by a constant current. Out of the four terminals, the terminal 1 and the terminal 3 in a pair face each other, and the terminal 2 and the terminal 4 in a pair face each other.
It is to be noted that the resistances of the four resistors (R1, R2, R3, R4) are not fixed values in the model of the Hall element illustrated in FIGS. 1A and 1B. In the case where the Hall elements are formed as an N-well in a semiconductor substrate, there is generally a density distribution of an impurity concentration within the respective Hall elements, due to a process gradient in manufacturing semiconductors. Therefore, the voltage distribution within the Hall element (N-well) varies depending on the terminal from which the driving current is injected, out of the four terminals (terminal 1, terminal 2, terminal 3, terminal 4), and thus, the occurrence state of the depletion layer within the Hall element (N-well) varies. Therefore, each of the resistances of the four resistors R1, R2, R3, R4 in the model of the Hall element varies depending on the terminal from which the driving current is injected, out of the four terminals (terminal 1, terminal 2, terminal 3, terminal 4).
In FIGS. 1A and 1B, voltage signals Vhall (φ1) and Vhall (φ2) measured when the phase of the chopper clock is φ1 (the driving direction of the Hall element is 0 degree) and the phase of the chopper clock is φ2 (the driving direction of the Hall element is 90 degrees), respectively, are represented as the sum of the Hall electromotive force signal Vsig(B) corresponding to the target magnetic field B to be detected by the magnetic sensor using the Hall element and the offset voltage Vos(Hall) of the Hall element, as represented by Expression 1.
In this situation, by periodically switching the direction of a bias current of the Hall element between 0 degree and 90 degrees according to the phase of the chopper clock, it is possible to invert the polarity of the Hall electromotive force signal Vsig (B) corresponding to the target magnetic field to be detected, thereby it is possible to perform frequency modulation of the signal Vsig (B) at the frequency f_chop of the chopper clock. On the other hand, as for the DC offset voltage Vos(Hall) of the Hall element, even if the driving direction of the Hall element is switched between 0 degree and 90 degrees, the DC offset Vos(Hall) retains its polarity. Therefore, Vos(Hall) is not frequency-modulated by the chopper clock.
                    ⁢          EXPRESSION      ⁢                          ⁢      1      ⁢              :              ⁢                      SIGNAL    ⁢                  ⁢    GENERATED    ⁢                  ⁢    IN    ⁢                  ⁢    HALL    ⁢                  ⁢          ELEMENT      ⁢                          ⁢                          [              Math        .                                  ⁢        1            ]            {                                                      Vhall              ⁢                                                          ⁢                              (                                  ϕ                  ⁢                                                                          ⁢                  1                                )                                      =                                          +                                  Vsig                  ⁡                                      (                    B                    )                                                              +                                                Vos                  ⁡                                      (                    Hall                    )                                                  ⁢                                                                  ⁢                                  (                                                            WHEN                      ⁢                                                                                          ⁢                      CHOPPER                      ⁢                                                                                          ⁢                      CLOCK                                        =                                          ϕ                      ⁢                                                                                          ⁢                      1                                                        )                                                                                                                    Vhall              ⁢                                                          ⁢                              (                                  ϕ                  ⁢                                                                          ⁢                  2                                )                                      =                                          -                                  Vsig                  ⁡                                      (                    B                    )                                                              +                                                Vos                  ⁡                                      (                    Hall                    )                                                  ⁢                                                                  ⁢                                  (                                                            WHEN                      ⁢                                                                                          ⁢                      CHOPPER                      ⁢                                                                                          ⁢                      CLOCK                                        =                                          ϕ                      ⁢                                                                                          ⁢                      2                                                        )                                                                        
FIGS. 2A to 2D are views illustrating a signal waveform generated in the Hall element illustrated in FIGS. 1A and 1B mentioned above.
Based on the foregoing, in a case where the driving current of the Hall element is switched between 0 degree and 90 degrees according to the phase of the chopper clock, the signal Vhall generated in the Hall element exhibits a waveform as illustrated in FIGS. 2A to 2D.
Furthermore, FIG. 3 is a view illustrating a signal spectrum of the signal Vhall generated in the Hall element. Since the spectrum of the signal generated in the Hall element exhibits the spectrum as illustrated in FIG. 3, it is understood that the Hall electromotive force signal Vsig (B) corresponding to the target magnetic field to be detected and the offset voltage Vos(Hall) of the Hall element are separated in the frequency domain. This is the principle of the offset cancellation of the Hall element based on the Spinningcurrent method.
Furthermore, PTL1 for example, describes an order sequence in Spinningcurrent method, in which the direction of the driving current is switched in the clockwise direction in one Hall element out of two Hall elements and the driving current is switched in the counterclockwise direction in the other Hall element.
Furthermore, PTL3 relates to a chop Hall sensor configured to alternately switch a Hall element excitation current from a flow in one direction to a flow in another direction, and especially describes a Hall sensor in which a chopped sample-and-hold Hall-voltage circuit is clocked synchronously with the switched Hall element.
Furthermore, NPL2 describes a circuit system which time-discretizes (sampling) such as sample-and-hold, as a circuit configuration implementing the Spinningcurrent method of the Hall element.